Wavelength measurement direction for transverse and longitudinal waves

Steps:

• Transverse waves have particle displacement perpendicular to propagation; longitudinal waves have it parallel.
• Both waves propagate and transfer energy in the same direction along the spring.
• Displacement for transverse is perpendicular to that for longitudinal, so measured at right angles.
• Wavelength for both is the distance between consecutive similar points along the propagation direction.

Why D is correct:

• Wavelength is defined as the spatial period along the propagation direction for all waves, per wave equation λ = v/f, so measurements align parallel, not at right angles.

Why the others are wrong:

• A: Displacement in transverse is perpendicular to propagation, while in longitudinal it is parallel, making measurements at right angles.
• B: Energy propagates with the wave in the direction of travel for both types.
• C: Both waves travel in the same specified direction, so velocities align.

Final answer: D